The present invention is related to liquid crystal display devices, and, in particular to active matrix liquid crystal display devices.
A liquid crystal display device changes the direction of orientation of liquid crystals by applying an electric field to a layer of liquid crystals held in between two substrates, and carries out display using the changes in the optical characteristics of the liquid crystal layer resulting from such changes in the direction of orientation of the crystals. In a conventional active matrix type liquid crystal display, as is typified by the twisted nematic (TN) display mode in which the display is made utilizing the optical rotation characteristics of liquid crystals, the orientation of the electric field applied to the liquid crystal was set to be almost perpendicular to the substrate boundary surface. On the other hand, the method of carrying out display using the birefringence characteristics of liquid crystals (the in-plane switching mode) by making the orientation of the electric field applied using comb-tooth shaped electrodes to the liquid crystal almost parallel to the substrate, has been proposed, for example, in Japanese Patent Publication No. Sho 63-21907, and Japanese Unexamined Patent Publication No. Hei 5-505247. This in-plane switching mode has the advantage of wider viewing angles compared to the conventional TN mode and is a very promising technology for active matrix type liquid crystal display devices.
As liquid crystal materials for active matrix type liquid crystal display devices of the in-plane switching mode, it has been proposed to use liquid crystal mixtures having relatively low specific resistances (Japanese Unexamined Patent Publication No. Hei 7-306417), liquid crystal mixtures containing 4-(cyclohexylcarbonyloxy)-benzonitrile in order to achieve both low driving voltage and high response speed (Japanese Unexamined Patent Publication No. Hei 9-125063), liquid crystal mixtures containing chemical compounds having fluorine as a polar group (Japanese Unexamined Patent Publication Nos. Hei 9-85541 and Hei 9-181823), or liquid crystal materials containing constituents with cyano group (D. Klement et al, SID International Symposium 98, 26.3), etc.
Further, in this in-plane switching mode, the relationships given by the following equations [Eqn. 1] and [Eqn. 2] are known to exist between the driving voltage, the liquid crystal response time, and the physical properties of the liquid crystal material (Masahito Oh-e and Katsumi Kondo, Applied Physics Letters, Vol. 67, pp. 3895-3897, 1995; Masahito Oh-e and Katsumi Kondo, Applied Physics Letters, Vol. 69, pp. 623-625, 1996).
xcfx84offxe2x88x9dxcex31xc3x97d2/K22xe2x80x83xe2x80x83[Eqn. 1]
Vthxe2x88x9d(Lxc3x97{square root over ((K+L 22/xcex94∈)))}/dxe2x80x83xe2x80x83[Eqn. 2]
Here, Vth is the threshold voltage of the liquid crystal, K22 is the twist elastic constant of the liquid crystal material, xcex94∈ is the dielectric anisotropy, L is the electrode spacing (see FIG. 1), d is the thickness of the liquid crystal layer (see FIG. 1) xcfx84off is the response time of the liquid crystals from the voltage applied condition to the no-voltage condition, and xcex31 is the rotational viscosity of the liquid crystals.
Further, it is possible to transform [Eqn. 1] and [Eqn. 2] respectively into [Eqn. 3] and [Eqn. 4] because dxc3x97xcex94n is almost constant in order to maintain the optical characteristics.
xcfx84offxe2x88x9dxcex31/(K22xc3x97xcex94n2)xe2x80x83xe2x80x83[Eqn. 3]
Vthxe2x88x9dLxc3x97xcex94nxc3x97{square root over ((K+L 22/xcex94∈))}xe2x80x83xe2x80x83[Eqn. 4]
As can be seen from these equations, the response time xcfx84off becomes shorter as the viscosity of the liquid crystal xcex31 becomes lower, and the driving voltage becomes lower as the dielectric anisotropy xcex94∈ becomes larger. However, in the case of most liquid crystal materials, there is an almost proportional relationship between the viscosity and the dielectric anisotropy xcex94∈, that is, there is a trend that the viscosity is lower in liquid crystals with smaller xcex94∈ and the viscosity becomes higher as xcex94∈ becomes larger. This is because there is a trend that the dipole moments of high polar liquid crystal molecules which make xcex94∈ of mixtures large are large, and the intermolecular interaction between molecules is large in materials with large dipole moment, and consequently, the viscosity of the entire liquid crystal becomes large. Therefore, in the in-plane switching mode of display, there is a trade-off between the high-speed response characteristics of liquid crystals and low driving voltage. In other words, if a large amount of low polarity component with xcex94∈xe2x89xa61 and relatively low viscosity, that is, a so called neutral component, is added, although the viscosity gets reduced and a fast response can be achieved, the driving voltage also increases at the same time. Further, if a large amount of high polar component with large xcex94∈ is added, although the driving voltage can be reduced, the viscosity increases thereby making the response of the liquid crystal slower. Furthermore, not much has so far been proposed about the method of controlling the twist elastic constant K22 which is one additional parameter affecting the driving voltage and the response time.
On the other hand, in order to achieve high contrast, a number of technologies have been developed for placing the spacers to keep the spacing between the pair of substrates constant in a non-displaying region of the display device. For example, such methods have been proposed as described in Japanese Unexamined Patent Publication No. Hei 10-170928, Japanese Unexamined Patent Publication No. Hei 9-61828, Japanese Unexamined Patent Publication No. Hei 6-250194, Japanese Unexamined Patent Publication No. Hei 5-53121, Japanese Unexamined Patent Publication No. Hei 5-173147, Japanese Unexamined Patent Publication No. Hei 8-160433, Japanese Unexamined Patent Publication No. Hei 8-292426, and Japanese Unexamined Patent Publication No. Hei 7-325298, etc.
As has been described above, in the liquid crystal materials for active matrix type liquid crystal display devices using the in-plane switching mode, there is a trade-off relationship between the response time and the driving voltage of the liquid crystals, that is, the driving voltage increases if the response time is decreased by reducing the viscosity of the liquid crystals by increasing the neutral liquid crystal component and the response time decreases if the dielectric anisotropy xcex94∈ is made large, and hence there was the problem that it was difficult to achieve both lower viscosity and higher xcex94∈ of the liquid crystal material, that is, to achieve both high-speed response and low driving voltage. In addition, so far the method of controlling the twist elastic constant K22 of the liquid crystals was not clear.
Further, from the results of experiments, it was found that there is a trade-off relationship between high response speed and high contrast in the in-plane switching mode active matrix type liquid crystal display devices. It was also found that when the content of the neutral component in the liquid crystal layer was increased thereby attempting to obtain a high response speed due to reduced viscosity, there was a reduction in the contrast, and it is because the brightness at the black state increased. In active matrix type liquid crystal display devices using the in-plane switching mode, normally, polarizers placed so that the polarization axes are approximately at right angles are used as the optical means for changing the optical characteristics in accordance with the molecular orientation of the liquid crystal layer. In this case, the transmittance increases as the voltage applied to the liquid crystal layer is increased, that is, the normally closed mode is used. In the case of this normally closed display mode, the orientation of the liquid crystal molecules around the spacers for keeping the spacing between the substrates constant differs from the direction of controlled orientation of the liquid crystal molecules near the substrate, and hence light leaks around the spacers at the black state thereby increasing the black brightness and consequently reducing the contrast. From the results of further investigations, it was found that, as the content of the neutral component in the liquid crystal layer is increased, this light leak around the spacers increases, the brightness at the black state increases, and as a result, the contrast decreases.
In view of the above problems in the conventional technology, the first objective of the present invention is to provide an active matrix type liquid crystal display device using the in-plane switching mode in which both high response speed and high contrast are achieved. The second objective of the present invention is to provide an active matrix type liquid crystal display device using the in-plane switching mode in which both high response speed and high contrast are achieved, while also achieving a low driving voltage.
In order to achieve the first objective mentioned above, the liquid crystal display device according to the present invention is a liquid crystal display device having a pair of substrates whose spacing is determined by spacers, a liquid crystal layer filled in the space between said pair of substrates, a set of electrodes formed on the surface of one of the substrates of said pair of substrates for applying an electric field to said liquid crystal layer, and a pair of optical polarizers with mutually perpendicular axes of polarization placed so that they enclose said liquid crystal layer, with said liquid crystal display device having the characteristic that, said spacers are in a non-displaying area, said liquid crystal layer contains 40% or more weight percentage but 100% or less weight percentage of a constituent component with a dielectric anisotropy of xcex94∈xe2x89xa61, and the directions of controlled orientation of liquid crystal molecules at the two surfaces between said liquid crystal layer and said pair of substrates are almost parallel to each other, and the polarization axis of one of the polarizers is almost the same as the direction of controlled orientation of liquid crystal molecules at said surface.
In addition, in order to achieve said objective, the liquid crystal display device according to the present invention has a pair of substrates whose spacing is kept constant by spacers, a liquid crystal layer filled in the space between said pair of substrates, a set of electrodes formed on the surface of one of the substrates of said pair of substrates for applying an electric field to said liquid crystal layer, and a pair of optical polarizers with mutually perpendicular axes of polarization placed so that they enclose said liquid crystal layer, with said liquid crystal display device having the characteristic that, the rotational viscosity coefficient xcex31 and the birefringence xcex94n of said liquid crystal layer satisfy the condition of 1xc3x97103 mPaxc2x7sxe2x89xa6xcex31/xcex94n2xe2x89xa61.2xc3x97104 mPaxc2x7s.
Further, in the liquid crystal display device having a pair of substrates whose spacing is kept constant by spacers, a liquid crystal layer filled in the space between said pair of substrates, a set of electrodes formed on the surface of one of the substrates of said pair of substrates for applying an electric field to said liquid crystal layer, and a pair of optical polarizers with mutually perpendicular axes of polarization placed so that they enclose said liquid crystal layer, said liquid crystal display device has the characteristic that, said spacers are in a non-displaying area, said liquid crystal layer contains 40% or more weight percentage but 100% or less weight percentage of a constituent component with a dielectric anisotropy of xcex94∈xe2x89xa61, the rotational viscosity coefficient xcex31 and the birefringence xcex94n of said liquid crystal layer satisfy the condition of 1xc3x97103 mPaxc2x7sxe2x89xa6xcex31/xcex94n2xe2x89xa61.2xc3x97104 mPaxc2x7s, the directions of controlled orientation of liquid crystal molecules at the two surfaces between said liquid crystal layer and said pair of substrates are almost parallel to each other, and the polarization axis of one of the polarizers is almost the same as the direction of controlled orientation of liquid crystal molecules at said surface.
It is preferable that the content of said neutral component is 40% or more weight percentage but 90% or less weight percentage.
Further, it is preferable that the rotational viscosity coefficient xcex31 and the birefringence xcex94n of said liquid crystal layer satisfy the condition of 1xc3x97103 mPaxc2x7sxe2x89xa6xcex31/xcex94n2xe2x89xa66xc3x97103 mPaxc2x7s.
In addition, the set of electrodes of this liquid crystal display device is a set consisting of pixel electrodes, common electrodes, and active devices.
In addition, the active devices in this liquid crystal display device are thin-film transistors.
Further, the at least either one of the pixel electrodes and the common electrodes of this liquid crystal display device are formed as transparent electrodes.
In this liquid crystal display device, the birefringence xcex94n and the thickness d of said liquid crystal layer satisfy the condition of 0.2 xcexcm less than dxc2x7xcex94n less than 0.4 xcexcm.
Further, the spacers in this liquid crystal display device are structural components formed on one of the substrates.
At least one of the constituent components with a dielectric anisotropy of xcex94∈xe2x89xa61 contained in said liquid crystal layer can be a chemical compound having two ring structures in the molecule, and said ring structure is a combination of a benzene ring and a cyclohexane ring. Or else, at least one of the constituent components with a dielectric anisotropy of xcex94∈xe2x89xa61 contained in said liquid crystal layer can be a chemical compound having only one ring structure in the molecule, and said ring structure is either a benzene ring or a cyclohexane ring.
In order to achieve said second objective, the liquid crystal display device according to the present invention has the characteristic that its liquid crystal layer contains a chemical compound having the structure indicated by the following chemical formula in the molecule. 
(X1 and X2 in this chemical formula denote H or F.)
Further, there is the characteristic that the liquid crystal layer contains an medium polar component between a low polar component with a dielectric anisotropy of xcex94∈xe2x89xa61 and the high polar component expressed by above chemical formula. The liquid crystal component with medium polar can also be a liquid crystal component with a structure selected from the set expressed by the following formula. 
(X1 and X2 in this chemical formula denote H or F. A denotes either a benzene ring or a cyclohexane ring.)
Further, the spacing L between the pixel electrodes and the common electrodes, the birefringence xcex94n of said liquid crystal layer, and the dielectric anisotropy xcex94∈ satisfy the condition of Lxcex94n/{square root over ( )}xcex94∈xe2x89xa60.55 xcexcm. In addition, it is preferable that the condition Lxcex94n/{square root over ( )}xcex94∈xe2x89xa60.4 xcexcm is satisfied.
Further, at least either one of the pixel electrodes and the common electrodes are made of a transparent material, and the birefringence xcex94n and the dielectric anisotropy xcex94∈ satisfy the condition of xcex94n/{square root over ( )}xcex94∈5.5xc3x9710xe2x88x922.
In addition, in the liquid crystal display device according to the present invention, the liquid crystal layer has a dielectric anisotropy of 7 or more and a twist elastic constant K22 of 5.5 pN or less.
In addition, the liquid crystal display device according to the present invention has the characteristic that the response time between the lowest brightness level and the highest brightness level is less than or equal to one frame period. Also, it is preferable that the response time between gray levels is less than or equal to one frame period.